1. Field of Invention
The present invention relates to a fuel cell system, particularly to a technology to optimize feed rate of fuel gas and oxidizer gas to a fuel cell in a fuel cell system.
2. Description of Related Art
Fuel cell is an electrochemical device which converts the chemical energy of a chemical reaction directly into electrical energy. A typical fuel cell consists of an electrolyte membrane and anode and cathode compartments sandwiching the electrolyte membrane therebetween, in which fuel gas is fed continuously to the anode compartment, oxidizer gas is fed continuously to the cathode compartment, and oxygen from the oxidizer gas and hydrogen contained in the fuel gas electrochemically reacts to generate electric power. The fuel cells are classified into alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) and polymer electrolyte fuel cell (PEFC), depending on the kind of electrolyte. Fuel cells of the latter two types are suitable as power sources for electric vehicles because of their solid electrolyte membranes easy in handling.
Generally, in the fuel cell used on ground level, air is used as the oxidizer gas. A typical air supply system of the fuel cell includes an air filter for removing dust contained in the air, a compressor for pressurizing the filtered air to a desired pressure, and pipings from the compressor to a fuel cell stack.
The air filter clogs due to a use in a dusty environment or a performance degradation of the compressor with passage of time induce an insufficient feed of reaction gas to the fuel cell, which causes lowering of an output voltage thereof. Also, the output voltage is lowered due to the change in the fuel cell stack itself with passage of time.
In the system shown in Japanese Patent Application Laid-Open Publication No. 2003-331895, a reference electrode is provided at an anode side of the electrolyte membrane, and by measuring an electrical potential difference between the reference electrode and the anode, a linear relation as a reference between the electrical potential difference and an output current is obtained. Then, the insufficient feed of reaction gas in the fuel cell is detected, when the relation between the electrical potential difference and the current measured during power generation deviates from the linear relation.
Japanese Patent Application Laid-Open Publication No. 10-326625 discloses a device in which when the output current of the fuel cell is drastically increased, the output current is limited to equal to or less than a current value corresponding to the reaction gas feed rate, and the reaction gas feed rate is controlled based on the current value thus limited.